Semiconductor device

ABSTRACT

Solved is a problem of attenuation of output amplitude due to a threshold value of a TFT when manufacturing a circuit with TFTs of a single polarity. In a capacitor ( 105 ), a charge equivalent to a threshold value of a TFT ( 104 ) is stored. When a signal is inputted thereto, the threshold value stored in the capacitor ( 105 ) is added to a potential of the input signal. The thus obtained potential is applied to a gate electrode of a TFT ( 101 ). Therefore, it is possible to obtain the output having a normal amplitude from an output terminal (Out) without causing the amplitude attenuation in the TFT ( 101 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/483,080, filed May 30, 2012, now allowed, which is a continuation ofU.S. application Ser. No. 12/630,869, filed Dec. 4, 2009, now U.S. Pat.No. 8,212,257, which is a continuation of U.S. application Ser. No.10/914,081, filed Aug. 10, 2004, now U.S. Pat. No. 7,629,612, which is acontinuation of U.S. application Ser. No. 10/198,693, filed Jul. 16,2002, now U.S. Pat. No. 6,774,419, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2001-243984 on Aug.10, 2001, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having thin filmtransistors (hereinafter, referred to as TFTs) formed on an insulatingsurface of glass, plastics, or the like. In particular, included in thesemiconductor devices are pulse output circuits such as a shift registercircuit, latch circuit, buffer circuit, and level shift circuit, andamplification circuits such as an amplifier, each being used as a drivercircuit of a display device.

2. Description of the Related Art

In recent years, a display device having a semiconductor thin filmformed on an insulator such as a glass substrate, in particular, anelectronic circuit manufactured with TFTs is used in various fields. Theelectronic circuit is often used in a display device. Active matrixdisplay devices such as an LCD (liquid crystal display) are used in manyproducts and widely spread. In the active matrix display device formedwith TFTs, several hundred thousands to several millions of pixels arearranged in a matrix form, and a charge at each pixel is controlled bythe TFT disposed at each pixel to thereby display an image.

As a further updated technique, a polysilicon TFT technique is beingdeveloped in which on the substrate a driver circuit is formed in aperipheral region of a pixel portion by using TFTs simultaneously withthe pixel portion including pixel TFTs which constitute pixels. Thisgreatly contributes to reduction in size and lowering of powerconsumption of the device, and accordingly such a display device isbecoming indispensable to a display unit and the like provided in amobile information terminal whose application field is notably widenedin recent years.

Incidentally, in recent years, the display device is adopted in adisplay unit of various electronic equipments, and its industrial fieldis steadily expanding. Recently, it has been actively adopted inrelatively inexpensive electronic equipments, and further reduction incosts is thus desired.

In general, in a semiconductor device, a CMOS circuit is adopted inwhich both n-channel TFTs and p-channel TFTs are used in combination. Adisplay device has a multilayer structure with manufacturing steps of:film formation; exposure with photomasks; and etching are repeated. Thesteps are extremely complicated, and manufacturing costs thus increase.In addition, in a case of integrally forming the driver circuit and thepixel portion on the substrate as described above, yield is intenselyaffected by the steps since the defect of a part leads to the defect ofa product as a whole.

A method of reducing manufacturing costs comprises reducing the numberof steps as much as possible and manufacturing a device in a simple wayas well as in a short period of time. Here, a display device ismanufactured not with the CMOS structure but with a structure with TFTsof a single polarity in which either n-channel TFTs or p-channel TFTsare used, as a driver circuit structure. Thus, the number of steps ofdoping an impurity which imparts a conductivity type to semiconductorlayers can be mathematically reduced to half, and further, the number ofphotomasks can also be reduced, which is effective to a great extent.Moreover, the manufacturing steps become simpler with a contribution toan improvement of yield.

FIG. 2 shows an example of an inverter formed of two n-channel TFTs. Theinverter is of a dual input type in which signals are inputted to gateelectrodes of TFTs 201 and 202, and an inverted signal of an inputsignal of one TFT is the input of the other TFT.

An operation of the inverter shown in FIG. 2 is now simply explained. Itshould be noted that in this specification, on explaining a structureand operation of a circuit, different names are appropriately given tothree electrodes of a TFT, that is, “gate electrode, input electrode,and output electrode” or “gate electrode, source region, and drainregion”. When the operation of the TFT is explained, a gate-sourcevoltage is considered in many cases. However, it is difficult to make arigid distinction between the source region and the drain region of theTFT due to its structure. If unified names are given thereto, confusionmay be caused on contrary. That is the reason why the different namesare used here. When the input/output of a signal is explained, theelectrodes are referred to as input electrode and output electrode. Whenthe gate-source voltage or the like of the TFT is explained, one of theinput electrode and the output electrode is referred to as sourceregion, and the other as drain region.

Further, “a TFT is ON” means a state in which the absolute value of thegate-source voltage of the TFT exceeds a threshold voltage with acurrent flowing between the source and the drain. On the other hand, “aTFT is OFF” means a state in which the absolute value of the gate-sourcevoltage of the TFT does not reach a threshold voltage with no currentflowing between the source and the drain. As to the threshold value, forthe sake of simple explanation, it is assumed that there is nofluctuation in respective TFTs. Threshold values of n-channel TFTs areuniformly set to VthN, and threshold values of p-channel TFTs areuniformly set to VthP.

First, when H level is inputted to an input terminal (In) and L level isinputted to an inverted input terminal (Inb), the TFT 201 is turned OFFand the TFT 202 is turned ON. Then, L level appears at an outputterminal (Out) and its voltage becomes VSS. On the other hand, when Llevel is inputted to the input terminal (In) and H level is inputted tothe inverted input terminal (Inb), the TFT 201 is turned ON and the TFT202 is turned OFF. Then, H level appears at the output terminal (Out).

At this time, a potential at the time when the output terminal (Out)becomes H level is considered.

In FIG. 2, when H level is inputted to the gate electrode of the TFT201, L level is inputted to the gate electrode of the TFT 202. Then, theTFT 201 is turned ON, the TFT 202 is turned OFF, and thus, the potentialof the output terminal (Out) begins to increase. When the potential ofthe output terminal (Out) reaches (VDD−VthN), the gate-source voltage ofthe TFT 201 becomes equal to the threshold value VthN. That is, at thismoment, the TFT 201 is turned OFF so that the potential of the outputterminal (Out) cannot increase any further.

A case is considered in which inverters are connected in a plurality ofstages, as shown in FIG. 12A. Among the inverters of FIG. 12A, only aninitial inverter (InvA) is of such a single input and single output typeas shown in FIG. 12B. Each of subsequent inverters (InvB) is of such adual input and single output type as shown in FIG. 12C in order tosuppress a shoot-through current at the time of the circuit operation asmuch as possible. It should be noted here that a gate electrode of a TFT1201 is connected to a high potential side power supply VDD and remainsin an ON state as long as the gate-source voltage of the TFT 1201becomes lower than the threshold value. Therefore, even when a TFT 1202is turned ON, it is possible to obtain L level output by setting acurrent ability of the TFT 1202 larger than that of the TFT 1201, thoughthe output does not become completely equal to VSS.

In such a case, even when an amplitude of the input signal is in a rangeof VDD to VSS, the amplitude may be attenuated after passing through thestages of inverters one after another due to an influence of thethreshold values of the TFTs 1201 and 1211, as shown in FIG. 12D.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a circuit which isformed of TFTs of a single polarity and which is capable of operatingwithout causing such amplitude attenuation of an output signal asdescribed above.

To solve the above problems, the present invention employs the followingmeasures.

In an inverter shown in FIG. 2, the cause of generation of outputamplitude attenuation is as follows. That is, when L level is inputtedto an input terminal (In) and H level is inputted to an inverted inputterminal (Inb), a potential applied to a gate electrode of a TFT 201 isequal to a potential on the input electrode side of the TFT 201, thatis, a high potential side power supply VDD. Therefore, a potential of anoutput terminal (Out) is only allowed to increase up to (VDD−VthN).

In other words, when H level appears at the output terminal (Out), inorder to obtain a state in which its potential is equal to VDD, thepotential applied to the gate electrode of the TFT 201 needs to behigher than VDD, or to be precise, higher than (VDD+VthN).

Therefore, in the present invention, to solve the above problems, acapacitor means is employed to store a charge equivalent to a thresholdvoltage of the TFT 201 in advance. When a signal is inputted thereto,the charge thus stored is added to the input signal, whereby thepotential applied to the gate electrode of the TFT 201 is raised to(VDD+VthN).

According to the present invention, there is provided a semiconductordevice comprising first to fourth transistors and a capacitor means,characterized in that:

the first to fourth transistors each have the same conductivity type;

a first electrode of the capacitor means is electrically connected to afirst signal input terminal, and a second electrode of the capacitormeans is electrically connected to a gate electrode of the firsttransistor;

a gate electrode of the second transistor is electrically connected to asecond signal input terminal;

an input electrode of the first transistor is electrically connected toa first power supply, and an output electrode of the first transistor iselectrically connected to a signal output terminal;

an input electrode of the second transistor is electrically connected toa second power supply, and an output electrode of the second transistoris electrically connected to the signal output terminal;

a gate electrode and an output electrode of the third transistor eachare electrically connected to the signal output terminal, and an inputelectrode of the third transistor is electrically connected to thesecond electrode of the capacitor means; and

a gate electrode and an output electrode of the fourth transistor eachare electrically connected to the second electrode of the capacitormeans, and an input electrode of the fourth transistor is electricallyconnected to the first electrode of the capacitor means.

In addition, according to the present invention, there is provided asemiconductor device comprising first to fourth transistors and acapacitor means, characterized in that:

the first to fourth transistors each have the same conductivity type;

a first electrode of the capacitor means is electrically connected to afirst signal input terminal, and a second electrode of the capacitormeans is electrically connected to a gate electrode of the firsttransistor;

a gate electrode of the second transistor is electrically connected to asecond signal input terminal;

an input electrode of the first transistor is electrically connected toa first power supply, and an output electrode of the first transistor iselectrically connected to a signal output terminal;

an input electrode of the second transistor is electrically connected toa second power supply, and an output electrode of the second transistoris electrically connected to the signal output terminal;

a gate electrode and an output electrode of the third transistor eachare electrically connected to the signal output terminal, and an inputelectrode of the third transistor is electrically connected to thesecond electrode of the capacitor means; and

a gate electrode of the fourth transistor is electrically connected tothe second electrode of the capacitor means, an input electrode of thefourth transistor is electrically connected to the first electrode ofthe capacitor means, and an output electrode of the fourth transistor iselectrically connected to the signal output terminal.

According to the present invention, the capacitor means is a capacitormeans storing a threshold voltage of the fourth transistor, and it ischaracterized in that the stored voltage is added to a potential of asignal inputted from the first signal input terminal, and the thusobtained potential is applied to the gate electrode of the firsttransistor. With this structure, a gate-source voltage of the firsttransistor is at least the threshold value all the time, making itpossible to obtain the output without causing the amplitude attenuation.

Further, according to the present invention, it is characterized in thatthe semiconductor device is consist of transistors of a single polarity,i.e., consist of only n-channel transistors or only p-channeltransistors. With this structure, it is possible to simplifymanufacturing steps of a display device.

In a display device of the present invention, the capacitor means may beformed of a capacitance between the gate electrode and the inputelectrode of the fourth transistor, or formed of two materials selectedfrom the group consisting of an active layer material, a material forforming a gate electrode, and a wiring material, and an insulating layerbetween the two materials.

In the display device of the present invention, it is characterized inthat a signal inputted to the second signal input terminal is obtainedby inverting the polarity of a signal inputted to the first signal inputterminal. With this structure, when a signal appearing at the outputterminal is either H level or L level, no shoot-through current isgenerated between a power supply VDD and a power supply VSS in acircuit, making it possible to reduce the consumption current.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams for explaining a circuit structure andoperation of an embodiment mode of the present invention;

FIG. 2 is a diagram for explaining an inverter formed of TFTs of asingle polarity and an operation thereof;

FIGS. 3A and 3B are diagrams for explaining a potential at each node atthe time of the circuit operation in the embodiment mode of the presentinvention;

FIG. 4 is a diagram showing an embodiment of the present invention witha different structure from that of the embodiment mode;

FIGS. 5A and 5B are diagrams for explaining sectional structures of abottom gate type TFT and a dual gate type TFT;

FIGS. 6A to 6G are diagrams showing examples of electronic devices towhich the present invention can be applied;

FIGS. 7A to 7C are diagrams showing an example of manufacturing steps ofa liquid crystal display device;

FIGS. 8A to 8C are diagrams showing an example of manufacturing steps ofa liquid crystal display device;

FIGS. 9A to 9C are diagrams showing an example of manufacturing steps ofan active matrix substrate including a circuit formed of p-channel TFTs;

FIGS. 10A and 10B are diagrams showing an example of manufacturing stepsof a light emitting device;

FIGS. 11A and 11B are diagrams showing an example of manufacturing stepsof a light emitting device;

FIGS. 12A to 12D are diagrams for explaining a structure in whichinverters consisting of TFTs of a single polarity are connected in aplurality of stages and an operation thereof; and

FIGS. 13A and 13B are diagrams showing an example of a driver circuit ofthe present invention, which is consisting of p-channel TFTs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

A fundamental circuit structure of the present invention is shown inFIG. 1A. The circuit operates in the same manner as an inverter shown inFIG. 2, and is of a dual input and single output type. A signal, whichis obtained by inverting the polarity of a signal inputted to an inputterminal (In), appears at an output terminal (Out).

The circuit is structured by TFTs 101 to 104 and a capacitor means 105.

An operation of the circuit is explained. In FIGS. 3A and 3B, apotential at each node at the time of the circuit operation is shown.First, when L level is inputted to a first input terminal (In1) and Hlevel is inputted to a second input terminal (In2), the TFT 102 isturned ON, and a potential of the output terminal (Out) begins todecrease toward VSS. At this point, the potential of the output terminal(Out) has not lowered yet to reach L level, and the TFT 103 thus remainsin an ON state. Then, a current flows from the output terminal (Out) tothe capacitor means 105, and a potential applied to a gate electrode ofthe TFT 104 increases. Therefore, the TFT 104 is also turned ON. As thepotential of the output terminal (Out) further decreases, a gate-sourcevoltage of the TFT 103 becomes equal to VthN, with the TFT 103 beingturned OFF. At this point, even when the TFT 104 is still in an ONstate, a charge accumulated in the capacitor means 105 is dischargedthrough the TFT 104. Then, a gate-source voltage of the TFT 104continuously decreases so that the TFT 104 will be turned OFF beforelong.

With this structure, a threshold voltage VthN of the TFT 104 is storedin the capacitor means 105. At the first input terminal (In1), L levelappears and its potential is VSS. Thus the potential applied to the gateelectrode of the TFT 101 is higher than VSS by a voltage stored in thecapacitor means 105. That is, the potential at this time applied to thegate electrode of the TFT 101 is (VSS+VthN). Since L level appears atthe output terminal (Out) and its potential is VSS, a gate-sourcevoltage of the TFT 101 is VthN, and the TFT 101 is turned OFF (FIG. 3A).

Further, an operation of the circuit is explained when H level isinputted to the first input terminal (In1) and L level is inputted tothe second input terminal (In2). First, at the second input terminal(In2), H level is switched to L level, and the TFT 102 is turned OFF. Onthe other hand, at the first input terminal (In1), L level is switchedto H level. At this time, the TFT 103 remains in an OFF state so that notransfer of the charge stored in the capacitor means 105 occurs. As tothe TFT 104, a potential of a source region thereof increases, whereas agate-source voltage is VthN as it stands, the TFT 104 remaining in anOFF state. Therefore, even when L level is switched to H level at thefirst input terminal (In1), the voltage between both electrodes of thecapacitor means 105 is still stored. Accordingly, since the potential ofthe first input terminal (In1) increases to VDD from VSS, the potentialapplied to the gate electrode of the TFT 101 increases to (VDD+VthN)from (VSS+VthN). Therefore, H level appears at the output terminal (Out)with its potential being equal to VDD (FIG. 3B).

In accordance with the operation descried above, it is possible tonormally obtain from the signal input having an amplitude in a range ofVDD to VSS, the output having the amplitude in the same range withoutthe amplitude attenuation. As a result, it is possible to manufacturethe semiconductor device formed of the TFTs of a single polarity withemploying the methods described above. This contributes to the reductionin the number of manufacturing steps as well as in manufacturing costs.

EMBODIMENTS

Embodiments of the present invention are described below.

Embodiment 1

FIG. 4 shows a circuit structure in which connections of the circuitshown in FIG. 1A are partially modified. In FIG. 1A, the outputelectrode of the TFT 104 is connected to the gate electrode of the TFT101, whereas it is connected to the output terminal (Out) in FIG. 4.

An operation of the circuit is the same as described in the embodimentmode so that no explanation thereof is given here. The gate electrode ofthe TFT 101 is now considered in the circuit structure. In the circuitshown in FIG. 1A, even after the TFT 103 is turned OFF, the charge canbe transferred through the TFT 104 to some extent. However, in thecircuit shown in FIG. 4, no transfer passage exists for the chargeaccumulated in the gate electrode of the TFT 101 when the TFT 103 isturned OFF. If a fluctuation is supposed to be generated in thresholdvalues of the TFTs which form the circuit, there is a possibility thatthe gate-source voltage of the TFT 101 does not sufficiently decrease tothe level equal to the threshold value of the TFT 101. In considerationof the above, by setting a current ability of the TFT 102 larger thanthat of the TFT 101, it is possible to obtain the normal L level outputeven when the TFT 101 is not completely turned OFF.

Embodiment 2

In the following, a method of simultaneously manufacturing TFTs ofdriving circuit portions provided in the pixel portion and the peripherythereof on the same substrate is described. Although the step ofmanufacturing a liquid crystal display device is shown in thisembodiment, the present invention is not limited to the liquid crystaldisplay device as mentioned above.

First, as shown in FIG. 7A, a base film 5002 made of an insulating filmsuch as a silicon oxide film, a silicon nitride film, or a siliconnitride oxide film is formed on a substrate 5001 made of glass such asbarium borosilicate glass or alumino borosilicate glass, typified by#7059 glass or #1737 glass of Corning Inc. For example, not shown infigures particularly, a silicon nitride oxide film fabricated from SiH₄,NH₃ and N₂O by a plasma CVD method is formed with a thickness of 10 to200 nm (preferably 50 to 100 nm), and a hydrogenated silicon nitrideoxide film similarly fabricated from SiH₄ and N₂O is formed with athickness of 50 to 200 nm (preferably 100 to 150 nm) to form alamination.

Island-like semiconductor layers 5003 to 5005 are formed of acrystalline semiconductor film manufactured by using a lasercrystallization method on a semiconductor film having an amorphousstructure, or by using a known thermal crystallization method. Thethickness of the island-like semiconductor films 5003 to 5005 is setfrom 25 to 80 nm (preferably between 30 and 60 nm). There is nolimitation on the crystalline semiconductor film material, but it ispreferable to form the film from silicon or a silicon germanium (SiGe)alloy.

A laser such as a pulse oscillation type or continuous emission typeexcimer laser, a YAG laser, or a YVO₄ laser is used for manufacturingthe crystalline semiconductor film in the laser crystallization method.A method of condensing laser light emitted from a laser oscillator intoa linear shape by an optical system and then irradiating the light tothe semiconductor film may be employed when these types of lasers areused. The crystallization conditions may be suitably selected by theoperator, but the pulse oscillation frequency is set to 30 Hz, and thelaser energy density is set from 100 to 400 mJ/cm² (typically between200 and 300 mJ/cm²) when using the excimer laser. Further, the secondharmonic is utilized when using the YAG laser, the pulse oscillationfrequency is set from 1 to 10 kHz, and the laser energy density may beset from 300 to 600 mJ/cm² (typically between 350 and 500 mJ/cm²). Thelaser light which has been condensed into a linear shape with a width of100 to 1000 μm, for example 400 μm, is then irradiated over the entiresurface of the substrate. This is performed with an overlap ratio of 80to 98%.

Next, a gate insulating film 5006 is formed covering the island-likesemiconductor layers 5003 to 5005. The gate insulating film 5006 isformed of an insulating film containing silicon with a thickness of 40to 150 nm by a plasma CVD method or a sputtering method. A 120 nm thicksilicon nitride oxide film is formed in this embodiment. The gateinsulating film is not limited to such a silicon nitride oxide film, ofcourse, and other insulating films containing silicon may also be used,in a single layer or in a lamination structure. For example, when usinga silicon oxide film, it can be formed by the plasma CVD method with amixture of TEOS (tetraethyl orthosilicate) and O₂, at a reactionpressure of 40 Pa, with the substrate temperature set from 300 to 400°C., and by discharging at a high frequency (13.56 MHz) with electricpower density of 0.5 to 0.8 W/cm². Good characteristics of the siliconoxide film thus manufactured as a gate insulating film can be obtainedby subsequently performing thermal annealing at 400 to 500° C.

A first conductive film 5007 and a second conductive film 5008 are thenformed on the gate insulating film 5006 in order to form gateelectrodes. In this Embodiment, the first conductive film 5007 is formedfrom tantalum (Ta) with a thickness of 50 to 100 nm, and the secondconductive film 5008 is formed from tungsten (W) with a thickness of 100to 300 nm (FIG. 7A).

The Ta film is formed by sputtering, which a Ta target is sputtered byusing Ar. If an appropriate amount of Xe or Kr is added to the Ar duringsputtering, the internal stress of the Ta film will be relaxed, and filmpeeling can be prevented. The resistivity of an α phase Ta film is onthe order of 20 μΩcm, and the α phase Ta film can be used for the gateelectrode, but the resistivity of β phase Ta film is on the order of 180μΩcm and the β phase Ta film is unsuitable for the gate electrode. The αphase Ta film can easily be obtained if a tantalum nitride (TaN) film,which possesses a crystal structure near that of α phase Ta, is formedwith a thickness of 10 to 50 nm as a base for Ta in order to form the αphase Ta film.

The W film is formed by sputtering with W as a target. The W film canalso be formed by a thermal CVD method using tungsten hexafluoride(WF₆). Whichever is used, it is necessary to make the film low resistantin order to use it as the gate electrode, and it is preferable that theresistivity of the W film be set 20 μΩcm or less. The resistivity can belowered by enlarging the crystal grain of the W film, but for caseswhere there are many impurity elements such as oxygen within the W film,crystallization is inhibited, and the film becomes high resistant.Therefore, a W target having a purity of 99.9999% is thus used insputtering. In addition, by forming the W film while taking sufficientcare such that no impurities from the inside of the gas phase areintroduced at the time of film formation, a resistivity of 9 to 20 μΩcmcan be achieved.

Note that although the first conductive film 5007 and the secondconductive film 5008 are formed from Ta and W, respectively, in thisembodiment, the conductive films are not limited to these. Both thefirst conductive film 5007 and the second conductive film 5008 may alsobe formed from an element selected from the group consisting of Ta, W,Mo, Al, and Cu, or from an alloy material or a chemical compoundmaterial having one of these elements as its main constituent. Further,a semiconductor film, typically a polysilicon film, into which animpurity element such as phosphorous is doped, may also be used.Examples of preferable combinations other than that in this embodimentinclude: the first conductive film formed from TaN and the secondconductive film formed from W; the first conductive film formed from TaNand the second conductive film formed from Al; and the first conductivefilm formed from TaN and the second conductive film formed from Cu.

Next, a mask 5009 is formed from resist, and a first etching process isperformed in order to form electrodes and wirings. An ICP (inductivelycoupled plasma) etching method is used in this embodiment. A gas mixtureof CF₄ and Cl₂ is used as an etching gas, and a plasma is generated byapplying a 500 W RF electric power (13.56 MHz) to a coil shape electrodeat 1 Pa. A 100 W RF electric power is also applied to the substrate side(test sample stage), effectively applying a negative self-bias voltagethereto. The W film and the Ta film are both etched on the same orderwhen CF₄ and Cl₂ are mixed as the etching gas.

Edge portions of the first conductive layer and the second conductivelayer are made into a tapered shape by using a suitable resist maskshape and the effect of the bias voltage applied to the substrate sidewith the above etching conditions. The angle of the tapered portions isfrom 15 to 45°. The etching time may be increased by approximately 10 to20% in order to perform etching without leaving any residue on the gateinsulating film. The selectivity of a silicon nitride oxide film withrespect to a W film is from 2 to 4 (typically 3), and thereforeapproximately 20 to 50 nm of the exposed surface of the silicon nitrideoxide film is etched by this over-etching process. First shapeconductive layers 5010 to 5013 are thus formed of the first conductivelayers 5010 a to 5013 a and the second conductive layers 5010 b to 5013b by the first etching process. At this point, regions of the gateinsulating film 5006 not covered by the first shape conductive layers5010 to 5013 are made thinner by approximately 20 to 50 am by etching(FIG. 7B).

Then, a first doping process is performed to add an impurity element forimparting an n-type conductivity (FIG. 7B). Doping may be carried out byan ion doping method or an ion injecting method. The condition of theion doping method is that a dosage is 1×10¹³ to 5×10¹⁴ atoms/cm², and anacceleration voltage is 60 to 100 keV. As the impurity element forimparting the n-type conductivity, an element belonging to group 15,typically, phosphorus (P) or arsenic (As) is used, but phosphorus isused here. In this case, the conductive layers 5010 to 5013 become masksto the impurity element to impart the n-type conductivity, and firstimpurity regions 5014 to 5016 are formed in a self-aligning manner. Theimpurity element to impart the n-type conductivity in the concentrationrange of 1×10²⁵ to 1×10²¹ atoms/cm³ is added to the first impurityregions 5014 to 5016.

Next, as shown in FIG. 7C, a second etching process is performed. TheICP etching method is similarly used, so that CF₄, Cl₂ and O₂ are mixedwith an etching gas, and RF electric power of 500 W is supplied to thecoil shape electrode at a pressure of 1 Pa to generate plasma. RFelectric power of 50 W is supplied to the substrate side (test samplestage), and a lower self-bias voltage in comparison with the self-biasvoltage in the first etching process is applied to thereon. Anisotropicetching of a W film as the second conductive layer is performed undersuch a condition, and anisotropic etching of the Ta film as the firstconductive layer is performed at an etching speed slower than that ofthe anisotropic etching of the W film so that a second shape conductivelayers 5017 to 5020 (first conductive layers 5017 a to 5020 a and secondconductive layers 5017 b to 5020 b) are formed. A region of the gateinsulating film 5006 which is not covered with the second shapeconductive layers 5017 to 5020 is further etched by about 20 to 50 [nm]so that a thinned region is formed.

An etching reaction of the W film or the Ta film by the mixture gas ofCF₄ and Cl₂ can be guessed from a generated radical or ion species andthe vapor pressure of a reaction product. When the vapor pressures offluoride and chloride of W and Ta are compared with each other, thevapor pressure of WF₆ being fluoride of W is extremely high, and otherWCl₅, TaF_(s), and TaCl₅ have almost equal vapor pressures. Thus, in themixture gas of CF₄ and CO₂, both the W film and the Ta film are etched.However, when a suitable amount of O₂ is added to this mixture gas, CF₄and O₂ react with each other to form CO and F, and a large number of Fradicals or F ions are generated. As a result, an etching rate of the Wfilm having the high vapor pressure of fluoride is increased. On theother hand, with respect to Ta, even if F is increased, an increase ofthe etching rate is relatively small. Besides, since Ta is easilyoxidized as compared with W, the surface of Ta is oxidized by additionof O₂. Since the oxide of Ta does not react with fluorine or chlorine,the etching rate of the Ta film is further decreased. Accordingly, itbecomes possible to make a difference between the etching rates of the Wfilm and the Ta film, and it becomes possible to make the etching rateof the W film higher than that of the Ta film.

Then, as shown in FIG. 7C, a second doping process is performed. In thiscase, a dosage is made lower than that of the first doping process andunder the condition of a high acceleration voltage, an impurity elementfor imparting the n-type conductivity is doped. For example, the processis carried out with an acceleration voltage set to 70 to 120 keV and ata dosage of 1×10¹³ atoms/cm², so that new impurity regions are formedinside of the first impurity regions formed into the island-likesemiconductor layers in FIG. 7B. Doping is carried out such that thesecond conductive layers 5017 b to 5020 b are used as masks to theimpurity element and the impurity element is added also to the regionsunder the first conductive layers 5017 a to 5020 a. In this way, secondimpurity regions 5021 to 5023 overlapping with the first conductivelayers are formed.

As shown in FIG. 8A, a third etching process is performed. Cl₂ is usedas an etching gas here, and the third etching process is performed byICP etching device. In this embodiment, the etching is performed for 70seconds under the condition that a gas flow rate of Cl₂ is set to 60sccm and RF electric power of 350 W is supplied to the coil shapeelectrode at a pressure of 1 Pa to generate plasma. RF electric power isalso supplied to the substrate side (test sample stage), substantiallythe negative self-bias voltage is applied thereof. According to thethird etching process, the first conductive layers are reduced wherebyforming third shape conductive layers 5024 to 5027 (first conductivelayers 5024 a to 5027 a and second conductive layers 5024 b to 5027 b).At this point, a part of the second impurity regions 5021 to 5023 isthird impurity regions 5028 to 5030, which are not overlapping with thefirst conductive layers.

Impurity regions are formed into the respective island-likesemiconductor layers by the above mentioned step. The third shapeconductive layers 5024 to 5027 overlapping with the island-likesemiconductor films function as a gate electrode of TFTs.

A step of activating the impurity elements added in the respectiveisland-like semiconductor layers for the purpose of controlling theconductivity type is conducted. This step is carried out by a thermalannealing method using a furnace annealing oven. In addition, a laserannealing method or a rapid thermal annealing method (RTA method) can beapplied. The thermal annealing method is performed in a nitrogenatmosphere having an oxygen concentration of 1 ppm or less, preferably0.1 ppm or less and at 400 to 700° C., typically 500 to 600° C. In thisembodiment, a heat treatment is conducted at 500° C. for four hours.However, in the case where a wiring material used for the third shapeconductive layers 5024 to 5027 is weak to heat, it is preferable thatthe activation is performed after an interlayer insulating film(containing silicon as its main constituent) is formed to protect thewiring line or the like.

Further, a heat treatment at 300 to 450° C. for 1 to 12 hours isconducted in an atmosphere containing hydrogen of 3 to 100%, and a stepof hydrogenating the island-like semiconductor layers is conducted. Thisstep is a step of terminating dangling bonds in the semiconductor layerby thermally excited hydrogen. As another means for thermalhydrogenation, plasma hydrogenation (using hydrogen excited by plasma)may be carried out.

Next, as shown in FIG. 8B, a first interlayer insulating film 5031 madeof a silicon nitride oxide film having a thickness of 100 to 200 nm isformed. A second interlayer insulating film 5032 made of an organicinsulating material formed thereon. Contact holes are then formed withrespect to the first interlayer insulating film 5031, the secondinterlayer insulating film 5032, and the gate insulating film 5006,films are formed by wiring material, respective wirings 5033 to 5036 areformed by patterning, and then, a pixel electrode 5037 is formed bypatterning.

Next, the film made from organic resin is used for the second interlayerinsulating film 5032. As the organic resin, polyimide, polyamide, acryl,BCB (benzocyclobutene) or the like can be used. Especially, since thesecond interlayer insulating film 5032 has rather the meaning offlattening, acryl excellent in flatness is desirable. In thisembodiment, an acryl film is formed to such a thickness that steppedportions formed by the TFTs can be adequately flattened. The thicknessis preferably made 1 to 5 μm (more preferably 2 to 4 μm).

In the formation of the contact holes, dry etching or wet etching isused, and contact holes reaching the n-type impurity regions 5014 to5016, a contact hole reaching a source signal line (not illustrated), acontact hole reaching a gate signal line (not illustrated); a contacthole reaching a power supply line, and contact hole reaching the gateelectrodes 5024 to 5026 (not illustrated) are formed, respectively.

Further, the wirings 5033 to 5036 are formed by patterning into adesired shape a film consisting of a three layer laminate in which a 100nm thick Ti film, a 300 nm thick Al film containing Ti, and a 150 nmthick Ti film are continuously formed by sputtering. Other conductivematerials may of course be used. The pixel electrode 5037 is formed of amaterial having a high reflectivity when a display device is of areflection type. In this case, it may be formed simultaneously with thewirings. On the other hand, in a case of a transmission type displaydevice, the pixel electrode is formed of a transparent conductivematerial such as indium tin oxide (ITO). In this specification, thesubstrate which has reached the state shown in FIG. 8B throughconducting steps is referred to as active matrix substrate.

Then, an opposing substrate 5038 is prepared. A light shielding film5039 is formed on the opposing substrate 5038. The light shielding filmis made of chrome (Cr) or the like with a thickness of 100 to 200 nm.

On the other hand, an opposing electrode 5040 is formed in the pixelportion. The opposing electrode is made of a transparent conductivematerial such as ITO. Further, a film thickness of the opposingelectrode is desirably 100 to 120 nm to keep the transmission of visiblelight high.

On the active matrix substrate and the opposing substrate, orientationfilms 5041 and 5042 are formed. It is preferred that the orientationfilms 5041 and 5042 have a thickness of 30 to 80 nm. As the orientationfilm, SE7792 manufactured by Nissan Chemical Industries, Ltd. may beused for example. By using an orientation film with a high pretiltangle, it is possible to suppress the generation of disclination at thetime of driving the liquid crystal display device driven with the activematrix method.

Then, the orientation films 5041 and 5042 are rubbed. Preferably, thedirection of rubbing shows the counterclockwise TN (twisted nematic)orientation when the liquid crystal display device is completed.

Though not particularly shown in Embodiment 2, spacers may be formed inpixels by dispersion or patterning, thereby improving cell gapuniformity. In Embodiment 2, a photosensitive resin film is formed andis then subjected to patterning to form the spacers with a height of 4.0μm.

Further, the active matrix substrate and the opposing substrate arebonded to each other with a sealant 5043. As the sealant, there is usedXN-21S manufactured by Mitsui Chemicals which is of a thermosettingtype. In the sealant, a filler is mixed. The height of the filler is 4.0μm. After the sealant is cured, the active matrix substrate and theopposing substrate are simultaneously cut into a desired size.

Subsequently, liquid crystal 5044 is injected. As the liquid crystalmaterial, one having the low viscosity is preferred in consideration ofhigh speed responsibility and the like. In Embodiment 2, nematic liquidcrystal is used with which the orientation control is easily performed.Needless to say, high speed responsive ferroelectric liquid crystal orantiferroelectric liquid crystal may be used.

After the injection of liquid crystal, an injection inlet is sealed withUV setting resin or the like. Then, a polarizing plate is attachedemploying a known method. Finally, a connector (flexible printedcircuit: FPC) is mounted with which terminals drawn from elements orcircuits formed on the substrate and external signal terminals areconnected, thereby completing the product (FIG. 8C). In thisspecification, the product in such a state that it is ready for shipmentas described above is referred to as liquid crystal display device.

In accordance with the steps shown in Embodiment 2, only four photomasksare required to form the active matrix substrate (that is, island-likesemiconductor layer pattern, first wiring pattern (gate wiring,island-like source wiring, and capacitor wiring), contact hole pattern,and second wiring pattern (which includes pixel electrode and connectionelectrode). As a result, the number of steps can be reduced,contributing to the reduction in manufacturing costs and improvement ofyield.

In Embodiment 2, the top gate type TFT is explained as an example. Inaddition, the embodiment can be implemented by using a bottom gate typeTFT including a gate electrode formed below an active layer, as shown inFIG. 5A, or a dual gate type TFT containing gate electrodes verticallylocated so as to sandwich an active layer therebetwen, as shown in FIG.5B.

Embodiment 3

The steps shown in Embodiment 2 are explained as an example of a case inwhich pixels and peripheral driver circuits are formed of n-channelTFTs. However, it is possible to implement the present invention byusing p-channel TFTs.

In the case of n-channel TFTs, an impurity region, which is calledoverlap region, is provided in a region overlapping a gate electrode toinhibit the hot carrier degradation etc. On the other hand, in the caseof p-channel TFTs, an influence due to the hot carrier degradation issmall so that there is no need to particularly provide the overlapregion. In this case, the pixels and peripheral driver circuits can bemanufactured through simpler steps.

As shown in FIG. 9A, according to Embodiment 2, a base film 6002 isformed on an insulating substrate 6001 made of glass or the like. Then,island-like semiconductor layers 6003 to 6005, a gate insulating film6006, and conductive layers 6007 and 6008 are formed thereon. Althoughthe conductive layers 6007 and 6008 are laminated here, they may be acomposed of a single layer.

Then, as shown in FIG. 9B, resist masks 6009 are formed to conduct firstetching processing. In Embodiment 2, anisotropic etching is performedwith utilization of a selection ratio between materials of the laminatedconductive layers. However, since there is no need to provide a regionfunctioning as an overlap region in this example, normal etching may bealternatively performed. At this time, in the gate insulating film 6006there is formed a region which is thinned by about 20 to 50 nm due toetching in comparison with other regions.

Subsequently, first doping processing is conducted to dope an impurityelement which imparts p-type conductivity to the island-likesemiconductor layers. Conductive layers 6010 to 6013 are used as masksagainst the impurity element, and impurity regions are formed in aself-aligning manner. Boron (B) or the like is representative of theimpurity element imparting p-type conductivity. In this example, theimpurity regions are formed by ion doping with diborane (B₂H₆) such thatthe semiconductor layers have the impurity concentration of 2×10²⁰ to2×10²¹ atoms/cm³.

The resist masks are removed to obtain the state shown in FIG. 9C. Fromthis stage, the pixels and peripheral driver circuits are manufacturedin accordance with the step shown in FIG. 8B of Embodiment 2 and thesubsequent steps. Thus, the present invention can be implemented byusing the p-channel TFTs.

The circuit structure is similar to the structure with the n-channelTFTs as shown in FIG. 1A. However, the power supply has differentconnections from those of FIG. 1A in which the high potential side powersupply VDD and the low potential side power supply VSS are switched.

Embodiment 4

In Embodiment 4, manufacturing steps of a light emitting device whichuses in a pixel portion a light emitting element such as anelectroluminescent (EL) element.

In accordance with the manufacturing steps shown in Embodiment 2, astate in which first and second interlayer insulating films are formedso far is obtained, as shown in FIGS. 8A and 8B.

Then, contact holes are opened, as shown in FIG. 10A. The contact holesare formed by dry etching or wet etching so as to reach the impurityregion, source signal line, gate signal line, current supply line, andgate electrode, respectively.

An anode 7001 of the EL element is formed by depositing a transparentconductive film represented by an ITO film and patterning it into adesired shape. A laminate film consisting of Ti layer, Al layercontaining Ti, and Ti layer is formed, and it is patterned into adesired shape, forming wiring electrodes 7002 to 7005 and a pixelelectrode 7006. The thickness of the respective layers may be the sameas in Embodiment 2. The pixel electrode 7006 is formed so as to overlapthe anode 7001 formed in the earlier stage, thereby establishing acontact therebetween.

Next, a third interlayer insulating film 7007 is formed by preparing aninsulating film made of, for example, an organic resin material such asacrylic and forming an opening portion at a position corresponding tothe anode 7001 of the EL element. It is preferred that the openingportion is formed to have gently tapered side walls. If the taperedshape of the side walls of the opening portion is not sufficientlygentle, degradation and cut step of the EL layer due to the existence ofsteps become serious problems so that attentions should be giventhereto.

After forming an EL layer 7008, a cathode 7009 of the EL element isformed of cesium (Cs) with a thickness of 2 nm or less and silver (Ag)with a thickness of 10 nm or less. By making the film of the cathode7009 of the EL element extremely thin, light generated at the EL layeris transmitted through the cathode 7009 to be emitted.

Subsequently, a protective film 7010 is formed for the protection of theEL element. Thereafter, attachment of an FPC and other operations areconducted, thus completing the light emitting device.

FIG. 10B shows a detailed structure of the EL element in the lightemitting device shown in FIG. 10A according to Embodiment 4. An anode7101 of the EL element is made of a transparent conductive filmrepresented by an ITO film. Reference numeral 7102 denotes an EL layercontaining a light emitting layer. A cathode of the EL element is madeof a Cs film 7103 and an Ag film 7104, each of which is formed extremelythin. Denoted by reference numeral 7105 is a protective film.

By making a region of the EL element on the cathode side extremely thin,light generated at the EL layer 7102 is transmitted through the Cs film7103 and the Ag film 7104 forming the cathode to be emitted upward. Thatis, the region where TFTs are formed does not overwhelm the area of thelight emitting surface so that the aperture ratio can be set to almost100%.

In this example, the emission direction of light faces the side wherethe cathode is formed. If it is not desired that the light transmissionis made toward the side of the anode made of ITO, it is preferred that asecond interlayer insulating film 7000 is formed of an opaque filmcolored in block or the like.

In the above steps, there is described the structure in which thecathode is formed just above the EL layer and the anode is formed justunder the EL layer. If the pixel electrode under the EL layer is made ofTiN etc., and the electrode above the EL layer is made of ITO etc., itis possible to arrange the anode just above the EL layer and the cathodejust under the EL layer.

The following structure may also be adopted, though the aperture ratiois slightly lowered. The anode is arranged just under the EL layer, thecathode is arranged just above the EL layer, the electrode under the ELlayer is made of ITO etc., and the electrode above the EL layer is madeof MaAg etc., which is different form Embodiment 4, whereby lightgenerated at the EL layer is emitted toward the substrate side where theTFTs are formed, or downward.

Embodiment 5

In Embodiment 5, steps of manufacturing a light emitting device in adifferent manner from that of Embodiment 4 are described.

In accordance with the manufacturing steps shown in Embodiment 2, thestate in which the first and second interlayer insulating films areformed so far is obtained, as shown in FIGS. 8A and 8B.

Then, contact holes are opened, as shown in FIG. 11A. The contact holesare formed by dry etching or wet etching so as to reach the n-typeimpurity region, source signal line, gate signal line, current supplyline, and gate electrode, respectively.

Next, wirings 7201 to 7204 are formed, and a pixel electrode 7205, whichserves as the anode of the EL element, is formed as a laminate filmwhich consists of Ti film, Al film containing Ti, Ti film, andtransparent conductive film.

Then, a third interlayer insulating film 7206 is formed by preparing aninsulating film made of, for example, an organic resin material such asacrylic and forming an opening portion at a position corresponding tothe anode 7205 of the EL element. It is preferred that the openingportion is formed to have gently tapered side walls. If the taperedshape of the side walls of the opening portion is not sufficientlygentle, degradation and cut step of the EL layer due to the existence ofsteps become serious problems so that attentions should be giventhereto.

After forming an EL layer 7207, a cathode 7208 of the EL element isformed of cesium (Cs) with a thickness of 2 nm or less and silver (Ag)with a thickness of 10 nm or less. By making the film thickness of thecathode 7208 of the EL element extremely thin, light generated at the ELlayer is transmitted through the cathode 7208 to be emitted.

Subsequently, a protective film 7209 is formed for protection of the ELelement. Thereafter, attachment of an FPC and other operations areconducted, thus completing the light emitting device.

FIG. 11B shows a detailed structure of the EL element in the lightemitting device shown in FIG. 11A according to Embodiment 5. An anode ofthe EL element is made of a metal film 7301 made of the laminate of Ti,Al, and Ti films, and a transparent conductive film 7302 represented byis an ITO film. Reference numeral 7303 denotes an EL layer containing alight emitting layer. A cathode of the EL layer is made of a Cs film7304 and an Ag film 7305, each of which is formed extremely thin.Denoted by reference numeral 7306 is a protective film.

In the light emitting device manufactured in accordance with Embodiment5, the aperture ratio can be advantageously set to nearly 100%, as inthe display device of Embodiment 4 described below. Further, whenforming wiring electrodes and pixel electrodes, it is possible toperform patterning on the metal film made of the laminate including Tifilm, Al film, and Ti film, and on the transparent conductive film witha common photomask. Thus, it is possible to reduce the number ofphotomasks and simplify the manufacturing steps.

In the above steps, there is described the structure in which thecathode is formed just above the EL layer, the anode is formed justunder the EL layer. If the pixel electrode under the EL layer is made ofTiN etc., and the electrode above the EL layer is made of ITO etc., itis possible to arrange the anode just above the EL layer and the cathodejust under the EL layer.

The following structure may also be adopted, though the aperture ratiois slightly lowered. The anode is arranged just under the EL layer, thecathode is arranged just above the EL layer, the electrode under the ELlayer is made of ITO etc., and the electrode above the EL layer is madeof MaAg etc., which is different form Embodiment 5, whereby lightgenerated at the EL layer is emitted toward the substrate side where theTFTs are formed, or downward.

Embodiment 6

The present invention can be implemented by using p-channel TFTs. InEmbodiment 6, the structure and operation are explained.

FIG. 13A shows a structure of a circuit. The circuit is a dual input andsingle output type inverter structured by TFTs 1301 to 1304 and acapacitor means 1305. A signal obtained by inverting the polarity of asignal inputted to an input terminal (In) appears at an output terminal(Out).

An operation of the circuit is explained. First, when H level isinputted to a first input terminal (In1) and L level is inputted to asecond input terminal (In2), then the TFT 1302 is turned ON, and thepotential of the output terminal (Out) begins to increase toward VDD. Atthis time, the potential of the output terminal (Out) does not reach ashigh as H level, the TFT 1303 thus remains in an ON state. A currentflows from the capacitor means 1305 toward the output terminal (Out), apotential applied to a gate electrode of the TFT 1304 decreases, and theTFT 1304 is also turned ON. As the potential of the output terminal(Out) further increases, a gate-source voltage of the TFT 1303 becomesequal to VthP, with the TFT 1303 being turned OFF. At this point, evenwhen the TFT 1304 is still in an ON state, a charge accumulated in thecapacitor means 1305 is discharged through the TFT 1304. Then, agate-source voltage of the TFT 1304 continuously decreases so that theTFT 1304 will be turned OFF before long.

With this structure, a threshold voltage VthP of the TFT 1304 is storedin the capacitor means 1305. At the first input terminal (In1), H levelappears and its potential is VDD. Thus the potential applied to the gateelectrode of the TFT 1301 is lower than VDD by a voltage stored in thecapacitor means 1305. That is, the potential applied to the gateelectrode of the TFT 1301 is (VDD−VthP) at this time. Since H levelappears at the output terminal (Out) and its potential is VDD, thegate-source voltage of the TFT 1301 is VthP, and the TFT 1301 is turnedOFF.

Further, an operation of the circuit is explained when L level isinputted to the first input terminal (In1) and H level is inputted tothe second input terminal (In2). First, at the second input terminal(In2), L level is switched to H level, and the TFT 1302 is turned OFF.On the other hand, at the first input terminal (In1), H level isswitched to L level. At this time, the TFT 1303 remains in an OFF stateso that no transfer of the charge stored in the capacitor means 1305occurs. As to the TFT 1304, a potential of a source region thereofdecreases, whereas a gate-source voltage is VthP as it stands, the TFT1304 remaining in an OFF state. Therefore, even when H level is switchedto L level at the first input terminal (In1), the voltage between bothelectrodes of the capacitor means 1305 is still stored. Accordingly,since the potential applied to the first input terminal (In1) decreasesto VSS from VDD, the potential applied to the gate electrode of the TFT1301 decreases to (VSS−VthP) from (VDD−VthP). Thus, L level appears atthe output terminal (Out) with its potential being equal to VSS.

In accordance with the operation descried above, even when the circuitis structured by p-channel TFTs, it is possible to normally obtain, fromthe signal input having an amplitude in a range of VDD to VSS, theoutput having the amplitude in the same range without the amplitudeattenuation.

Embodiment 7

The present invention can be applied to manufacturing the display deviceof various electric equipments. As such electronic equipments, there arepointed out a portable information terminal (electronic book, mobilecomputer, cellular phone of the like), a video camera, digital camera, apersonal computer, a television, a cellular phone and the like. Exampleof these are shown in FIGS. 6A to 6G.

FIG. 6A illustrates a liquid crystal display or OLED display constitutedby a casing 3001, a support stand 3002, a display portion 3003 or thelike. The present invention can be applied to the display portion 3003.

FIG. 6B illustrates a video camera constituted by a main body 3011, adisplay portion 3012, a audio input portion 3013, operation switches3014, a battery 3015, an image receiving portion 3016 or the like. Thepresent invention can be applied to the display portion 3012.

FIG. 6C illustrates a notebook personal computer constituted by a mainbody 3021, a casing 3022, a display portion 3023, a keyboard 3024 or thelike. The present invention can be applied to the display portion 3023.

FIG. 6D illustrates a portable information terminal, constituted by amain body 3031, a stylus 3032, a display portion 3033, operationswitches 3034, a external interface 3035 or the like. The presentinvention can be applied to the display portion 3033.

FIG. 6E illustrates an audio reproduction device, more specifically anaudio device mounted in a motor vehicle and constituted by a main body3041, a display portion 3042, operation switches 3043 and 3044 or thelike. The present invention can be applied to the display portion 3042.The invention may be applied to any of portable or home audio devicesother than the above-described audio device mounted in a motor vehicle.

FIG. 6F illustrates a digital camera constituted by a main body 3051, adisplay portion (A) 3052, an ocular portion 3053, operation switches3054, a display portion (B) 3055, a battery 3056 or the like. Thepresent invention can be applied to each of the display portion (A) 3052and the display portion (B) 3055.

FIG. 6G illustrates a cellular phone constituted by a main body 3061, anaudio output portion 3062, an audio input portion 3063, a displayportion 3064, operating switches 3065, an antenna 3066 or the like. Thepresent invention can be applied to the display portion 3064.

Note that the above-described devices of this embodiment are onlyexamples and that the invention is not exclusively applied to them.

With the circuit of the present invention, it is possible to normallyobtain, from the signal input having an amplitude in a range of VDD toVSS, the output having the amplitude in the same range without theamplitude attenuation. Therefore, it is possible to manufacture thedriver circuit of the display device with the TFTs of a single polarityin accordance with the methods described above. This contributes to thereduction in the number of the manufacturing steps and the lowering ofthe manufacturing costs.

What is claimed is:
 1. A semiconductor device comprising: first, second,third and fourth transistors and a capacitor; wherein the first, second,third and fourth transistors are the same conductivity type, wherein afirst electrode of the capacitor is electrically connected to a gate ofthe first transistor, wherein the first electrode of the capacitor iselectrically connected to a gate of the fourth transistor, wherein thefirst electrode of the capacitor is electrically connected to one ofsource and drain of the third transistor, wherein a second electrode ofthe capacitor is electrically connected to one of source and drain ofthe fourth transistor, wherein a gate of the second transistor iselectrically connected to a first signal input terminal, wherein one ofsource and drain of the first transistor is electrically connected to asignal output terminal, wherein the one of source and drain of the firsttransistor is electrically connected to one of source and drain of thesecond transistor, wherein the one of source and drain of the firsttransistor is electrically connected to the other of source and drain ofthe third transistor, and wherein the other of source and drain of thesecond transistor is electrically connected to a first power supplyline.
 2. The semiconductor device according to claim 1, wherein thesecond electrode of the capacitor is electrically connected to a secondsignal input terminal.
 3. The semiconductor device according to claim 1,wherein a gate of the third transistor is electrically connected to theother of source and drain of the third transistor.
 4. The semiconductordevice according to claim 1, wherein the one of source and drain of thefourth transistor is electrically connected to a second signal inputterminal.
 5. The semiconductor device according to claim 1, wherein theother of source and drain of the first transistor is electricallyconnected to a second power supply line.
 6. The semiconductor deviceaccording to claim 1, wherein the other of source and drain of thefourth transistor is electrically connected to the gate of the fourthtransistor.
 7. The semiconductor device according to claim 1, whereinsaid conductivity type is an n-channel type.
 8. A display devicecomprising the semiconductor device according to claim 1, wherein thedisplay device comprises a display element.
 9. A display modulecomprising the display device according to claim 8, wherein the displaymodule comprises a flexible printed circuit.
 10. Electric equipmentcomprising the display module according to claim 9, wherein the electricequipment includes at least one of a battery, an image receivingportion, an audio input portion, an audio output portion and antenna.11. A semiconductor device comprising: first, second, third and fourthtransistors and a capacitor; wherein the first, second, third and fourthtransistors are the same conductivity type, wherein a first electrode ofthe capacitor is directly connected to a gate of the first transistor,wherein the first electrode of the capacitor is electrically connectedto a gate of the fourth transistor, wherein the first electrode of thecapacitor is directly connected to one of source and drain of the thirdtransistor, wherein a second electrode of the capacitor is directlyconnected to one of source and drain of the fourth transistor, wherein agate of the second transistor is directly connected to a first signalinput terminal, wherein one of source and drain of the first transistoris directly connected to a signal output terminal, wherein the one ofsource and drain of the first transistor is electrically connected toone of source and drain of the second transistor, wherein the one ofsource and drain of the first transistor is electrically connected tothe other of source and drain of the third transistor, wherein the otherof source and drain of the second transistor is electrically connectedto a first power supply line, wherein the one of source and drain of thesecond transistor is directly connected to the signal output terminal,and wherein the other of source and drain of the third transistor isdirectly connected to the signal output terminal.
 12. The semiconductordevice according to claim 11, wherein the second electrode of thecapacitor is electrically connected to a second signal input terminal.13. The semiconductor device according to claim 11, wherein a gate ofthe third transistor is electrically connected to the other of sourceand drain of the third transistor.
 14. The semiconductor deviceaccording to claim 11, wherein the one of source and drain of the fourthtransistor is electrically connected to a second signal input terminal.15. The semiconductor device according to claim 11, wherein the other ofsource and drain of the first transistor is electrically connected to asecond power supply line.
 16. The semiconductor device according toclaim 11, wherein the other of source and drain of the fourth transistoris electrically connected to the gate of the fourth transistor.
 17. Thesemiconductor device according to claim 11, wherein said conductivitytype is an n-channel type.
 18. A display device comprising thesemiconductor device according to claim 11, wherein the display devicecomprises a display element.
 19. A display module comprising the displaydevice according to claim 18, wherein the display module comprises aflexible printed circuit.
 20. Electric equipment comprising the displaymodule according to claim 19, wherein the electric equipment includes atleast one of a battery, an image receiving portion, an audio inputportion, an audio output portion and antenna.